New and advanced differential gear and transmission systems are being developed by the automotive industry. These new systems often involve high energy requirements. Therefore, component protection technology must be developed to meet the increasing energy requirements of these advanced systems.
An example of a gear apparatus is the limited slip differential. Limited slip differentials are provided in many vehicles to counteract a commonly occurring situation where one of the two driving tires or wheels is essentially void of traction. This may be in loose soil, sand, mud, or ice. In the standard open differential, the driving tire void of traction receives all of the powertrain torque, but spins without moving the vehicle in the desired direction. In a limited slip differential, a mechanism divides, or shares, a portion of the torque delivered by the powertrain with both driving wheels. By sharing the available torque, a wheel with some traction receives enough torque to move the vehicle in the desired direction. In addition, high performance vehicles have so much torque that in certain turning situations the torque on one wheel exceeds the available traction and, thus, performance suffers. The limited slip differential shares the torque between both wheels, enhancing the performance of the vehicle.
Limited slip differentials have a variety of mechanisms to provide torque transfer from the input pinion gear to the axle shafts. A common mechanism is a multi-plate wet clutch that transfers torque from the differential carrier to the side gear. These multi-plate clutches typically have a set of friction plates of one material or facing material, and a set of steel plates. One set of plates is linked through some means to the differential carrier, while the other set of plates is linked through a similar means to the side gear. As the side gear drives the axle shaft, torque is therefore transferred to the axle shaft and thereby the wheel and tire of the vehicle. This then gives a motive force to the vehicle.
In operation, the limited slip differential friction and steel clutch plates spin at different speeds with respect to one another when in a vehicle turn or when traction to one wheel is reduced or void. The relative rotational speed of the clutch plates may range from near zero revolutions per minute to very high speeds of several hundred revolutions per minute. The clutch plates are operated in most cases by a biasing spring force that pushes the two sets of plates together, as well as the differential gear set separating force.
Limited slip differentials require that the lubricant for the rear axle have proper friction characteristics, and that the friction characteristics last for a sufficient elapsed mileage or duration. The proper friction characteristic is that the friction coefficient rises with increasing plate rotational speed, and falls with decreasing plate rotational speed.
To provide the proper friction characteristics and lifetime, certain additives may be added as a top treat to the gear lubricant. These additives can be selected from a wide range of friction modifiers and related compounds. However, a particularly effective additive will not stay in solution in a top treat.
For the purposes of this disclosure, the phrase “gear fluids” is intended to include, but is not limited to, the foregoing gear and transmission systems and applications.
Gear fluids formulated according to the present disclosure are suitably formulated to protect transmission and gear drive components in metal-on-metal contact situations. However, additives which provide such improvement are difficult to maintain dissolved in a concentrate for deliverance to a gear fluid.
In an embodiment, a gear fluid composition for extreme pressure applications is provided. The gear fluid contains an base oil component and a friction modifier mixture. The friction modifier includes                (i) at least one alkyl phosphonic acid diester of the formula:        
wherein R1 is a hydrocarbyl group containing from about 8 to about 24 carbon atoms, R2 and R3 are independently selected from a hydrocarbyl group containing from about 1 to about 8 carbon atoms;                (ii) at least one alkyl phosphonic acid monoester of the formula        
wherein R4 is a hydrocarbyl group containing from about 8 to about 24 carbon atoms, R5 is selected from hydrogen and a hydrocarbyl group containing from about 1 to about 8 carbon atoms; and                (iii) at least one amine salt of a partial ester of phosphoric acid represented by the formula        
                wherein each of R6 and R8 is, independently, a hydrocarbyl group, and        R7 is hydrogen or a hydrocarbyl group, and wherein the ratio of (i) to        (ii) ranges from about 3 to about 5.5.        
In another embodiment, there is provided a method of improving the solubility of friction modifier components in a friction modifier additive package. The method includes blending at least one alky phosphonic acid diester of the formula:
with at least one alkyl phosphonic acid monoester of the formula:
and at least one amine salt of a partial ester of phosphoric acid represented by the formula
wherein R1 and R4 are selected from a hydrocarbyl group containing from about 8 to about 24 carbon atoms; R2, R3 and R5 are independently selected from hydrogen and a hydrocarbyl group containing from about 1 to about 8 carbon atoms; each of R6, and R8 is, independently, a hydrocarbyl group; and R7 is hydrogen or a hydrocarbyl group. The ratio of the diester to the monoester of the alkyl phosphonic acids in the mixture ranges from about 3 to about 5.5 and the total acid number (TAN) of the alkyl phosphonic acid diester is up to about 15. The foregoing components are blended in an amount of the base oil component sufficient to stabilize substantially all of the alkyl phosphonic acid diester and monoester.
An advantage of the compositions and methods described herein is that the components of the additive package remain substantially solubilized or stabilized in the base oil component without the need for additional solubilizing additives so that the additive package is substantially free from additive drop-out. The additive package described herein also enables a high concentration of friction modifier component to be delivered to a gear fluid. Such additive packages are particularly suitable for a wide variety of gear and/or transmission applications including, but not limited to, automotive gears, industrial gears, stationary gears, rear axles, limited slip differentials, conventional differentials, and/or automatic and manual transmissions. Further, such additive packages are suitable for use in multi-plate differentials, cone clutch differentials, torsen differentials, and/or dog clutch differentials.